Introduction to the Theory of Computation. PWS Publishing. 1997. ISBN 0-534-94728-X. Pages 310–313 of section 9.1: Hierarchy theorems. Christos Papadimitriou
1977年,海倫·奎恩與皮塞提出皮塞-奎恩理論(Peccei–Quinn theory),引入一種假想的亞原子粒子軸子,試圖解決CP守恆問題。 Georgi, H.; Quinn, H.R.; Weinberg, S. Hierarchy of interactions in Unified Gauge Theories